1. Field of the Invention
The present invention relates to non-isolated power supplies for supplying a low D.C. voltage from a high A.C. voltage. Such power supplies, also called power converters, can be found in most electric household appliances, for example, washing machines, for the supply of low voltage circuits such as, for example, logic circuits (microprocessors, programmable logic), control circuits of electromechanical actuators (relays) or electronic actuators (triacs), etc . . .
The present invention more specifically relates to converters intended for providing two D.C. voltages, one of which is positive and the other one of which is negative with respect to a reference point also forming a reference for the A.C. power supply.
2. Discussion of the Related Art
FIG. 1 shows a first example of a conventional AC/DC converter with positive and negative outputs. This converter uses a transformer T, the primary winding L1 of which receives on two terminals E1 and E2 an A.C. voltage Vac (for example, mains 240 V/50 Hz or 110 V/60 Hz). The two terminals of secondary L2 of transformer T provide a low A.C. voltage Vi, the peak amplitude of which approximately corresponds to the values desired for output voltages V+ and Vxe2x88x92 of the converter. Each output voltage V+, Vxe2x88x92 is sampled across a capacitor, respectively C1, C2. Each capacitor is connected, in series with a diode, respectively D1, D2, to secondary winding L2 of the transformer. The anode of diode D1 and the cathode of diode D2 are connected to the high terminal of winding L2. The common node M of capacitors C1 and C2, which is the reference of voltages V+ and Vxe2x88x92, is connected to input terminal E2.
FIG. 2 shows a second conventional example of an AC/DC converter adapted to providing two output voltages V+ and Vxe2x88x92 of opposite signs. A high voltage capacitor Cx is connected via a resistor R of small value to input terminal E1. The second terminal of capacitor Cx is connected via rectifying diodes D1, D2 to low voltage capacitors C1 and C2, the second terminals of which are connected to a reference terminal M connected to terminal E2. A Zener diode DZ1, DZ2, is connected in parallel on each of capacitors C1, C2. Capacitor Cx has a small value as compared to that of each of capacitors C1, C2, to provide a high impedance to the variations of the input voltage.
The above-described converters are relatively simple to form due to the small number of components used. However, these converters have the major disadvantage of being bulky and expensive.
For the converter of FIG. 1, the high cost and the large bulk are due to the transformer. Further, non-negligible losses appear even in the absence of any flow (iron losses of the transformer).
For the converter of FIG. 2, the cost and the bulk are due to the capacitor that must withstand the voltage of the supply system. This converter further generates non-negligible losses.
The present invention provides a novel solution to make a converter from a high A.C voltage into two symmetrical low D.C. voltages, which overcomes the disadvantages of known solutions.
The present invention aims, in particular, at providing a novel solution with a reduced or minimized bulk.
The present invention also aims at a solution that respects electromagnetic compatibility constraints.
The present invention also provides a novel converter that automatically adapts to the current demand of the load. More specifically, the present invention provides a solution adapted to a large range of currents, that is, which enables, if necessary, providing substantially larger currents than conventional solutions, without generating significant losses.
The present invention further aims at improving the regulation of the converter output voltage.
To achieve these and other objects, the present invention provides a converter receiving an A.C. voltage between an input terminal and a reference terminal and providing two D.C. voltages of opposite polarities across a first and a second capacitor, a common terminal of which is connected to the reference terminal. This converter includes a first one-way switch connected between the input terminal and a node of the circuit; a third capacitor, connected in parallel with the first capacitor, between said node and the reference terminal; circuit means including a second switch for connecting the second capacitor in series with the third capacitor when the second switch is closed; and a control circuit for closing the first switch only at the beginning and at the end of halfwaves of given polarity, the second switch being open when the first switch is closed.
According to an embodiment of the present invention, the circuit means include a controllable current source to ensure the supply of a constant current.
According to an embodiment of the present invention, the second switch is connected between said node and the reference terminal.
According to an embodiment of the present invention, the first capacitor is connected to said node via a first diode, the third capacitor being connected via a third diode to the reference terminal.
According to an embodiment of the present invention, a second diode is connected between the second capacitor and the junction point of the third capacitor and of the third diode.
According to an embodiment of the present invention, the control circuit receives an information about the D.C. voltage across the first capacitor and controls the first switch so that this D.C. voltage is substantially constant.
According to an embodiment of the present invention, the first switch is controllable in voltage or current to be turned off or on and is able to block the mains voltage forwardly or in reverse when it is not controlled to the on state.
The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.